Imaging description

In ultrasound, anisotropism refers to the different echogenicity that can occur within tissues with a directional internal structure depending on the angle of insonation. The term is derived from the Greek aniso (meaning not the same) and tropos (to turn or reflect). The phenomenon was first described in tendons [1,2], but can also occur in the kidneys where the radial arrangement of nephrons and intervening tissues results in greater echogenicity from parts of the kidney where the nephrons are perpendicular to the ultrasound beam when compared to parts where the nephrons are parallel to the ultrasound beam [3, 4]. In practice, this can result in an apparent echogenic pseudotumor in the polar parts of the kidneys when the ultrasound beam is centered on the mid-kidney (Figure 44.1).

Importance

Anisotropic renal pseudotumor may be misinterpreted as a true echogenic renal mass, suggestive of either angiomyolipoma or renal cell carcinoma, and result in unnecessary additional workup and patient anxiety.

Typical clinical scenario

This pseudotumor is a technical artifact and so can potentially be seen in any patient undergoing ultrasound of the kidneys.

Differential diagnosis

The key to recognizing anisotropic renal pseudotumor at ultrasound is to compare the image with the apparent mass when the transducer is centered on the mid-kidney to an image obtained when the transducer is closer to a radial alignment with the polar part of the kidney – the anisotropic pseudotumor will not be visible on the latter image, unlike a true mass which should be equally visible on both. In addition, anisotropic renal pseudotumor typically has ill-defined margins and fades gradually into the surrounding tissues, unlike a true renal mass which frequently has well-defined margins.